In cable communication systems it is often necessary to transmit signals long distances over coaxial cables. The strength of the transmitted signals decreases in proportion to the length of the cable over which the signals are transmitted, necessitating amplification of the signals at repeated intervals along the cable to maintain adequate signal strength. The electronic amplifiers used to amplify the signal inherently distort the signal as they amplify it. Ideally, such distortion is to be eliminated or reduced to some acceptable minimum level.
Feedforward amplifiers are able to provide relatively distortion-free amplification by extracting from the amplified signal a signal component representative of the distortion introduced by the amplifier, phase inverting that component, and then recombining the phase inverted distortion component with the amplified signal. The phase inverted distortion component cancels the distortion component in the amplified signal, leaving a relatively distortion-free amplified signal for transmission along the cable. To avoid further distortion, the signals must coincide precisely in time when they are recombined. To ensure such coincidence, delay means are provided to delay the signals and thereby compensate for time delays which arise when the signals pass through the various electronic components in the amplifier.
Two major design criteria govern the selection of a suitable delay means for inclusion in a feedforward amplifier. First, the delay means must delay signals passing therethrough for a precise time interval. For feedforward amplifiers passing signals in the 50 MHz-400 MHz band, the appropriate delay is on the order of a few nanoseconds, but the precise time delay required will vary from one amplifier to another. The second major design criterion requires that the delay means be matched to the impedance of the transmission cable in order to minimize attenuation of the signal.
One prior art technique utilized a short length of coaxial cable for the delay means in a feedforward amplifier. Because the delay means was constructed of cable identical to that comprising the main transmission cable, excellent impedance matching was obtainable. However, in order to adjust the time delay provided by such a coaxial cable segment, it is necessary to trim short pieces from the segment on a trial and error basis until the time taken for a signal to pass through the cable segment equals the desired time delay. This technique is extremely cumbersome and is not economically adaptable to the large scale volume production of feedforward amplifiers. Furthermore, the relatively bulky nature of such cable segments (about six feet of cable are typically required in each delay means) makes it difficult to package the amplifier which is preferably made as small as possible. In addition, the time delay characteristics of a cable segment are subject to change with temperature and so the vital design criteria mentioned above may be upset under field operating conditions.
Another prior art technique provided a delay line comprising a silicon substrate having a sapphire spiral precisely etched thereon (termed a "stripline" in the art). Computer control techniques were used to carefully control the etching process to yield a spiral having precise time delay and impedance characteristics. Such etched spirals may in practice only be adjusted by shortening the spiral length (wire jumpers are used for this purpose) which, in turn, varies the spiral time delay. This technique does not, however, enable independent adjustment of phase or time delay characteristics within a range of frequencies--which may be accomplished with the present invention. Furthermore, the time delay and impedance matching characteristics of such spiral elements are subject to change with temperature which is undesirable as mentioned above.
An object of the present invention is to provide a tuneable electronic delay line for a feedforward amplifier. The delay line may be installed in a feedforward amplifier and then adjusted to vary either its time delay or impedance matching characteristics, or both, as may be required to suit the operating requirements of the individual amplifier circuit with which the delay line is to function.
A related object is to provide a delay line for a feedforward amplifier which may be easily mass produced and which may be easily adjusted under volume production conditions.